1. Field of the Invention
The present invention relates generally to the restoration of teeth, and more particularly, to methods and devices for improving the accuracy and simplifying the process of performing such restorations by machining a prosthetic, such as a crown or bridge.
2. Background Information
Numerous methods exist for the restoration of teeth by dentists, including the use of artificial tooth material (such as gold or porcelain) to form a cast-restoration or a metal-ceramic restoration (i.e., dental prosthetics such as crowns). Prosthetic crowns are typically used to repair decayed tooth structure where support from the original tooth structure is either marginal, or unavailable.
Known techniques for preparing a tooth to receive a crown are described in U.S. Pat. No. 5,813,859 in PCT application PCT/US98/00910, both entitled xe2x80x9cMethod And Apparatus For Tooth Restorationxe2x80x9d, the contents of which are hereby incorporated by reference in their entireties. As described therein, known techniques of tooth restoration are susceptible to numerous variables, some of which are within the dentist""s control and some of which are not. All of these variables can detrimentally influence the accuracy with which: (1) the tooth is prepared to receive the crown; (2) the crown is prepared for placement on the tooth; and (3) the manner by which the crown is fit to and fixed on the prepared tooth.
The quality of a prosthetic crown will vary based on the skill of the person who actually produces the crown (e.g., laboratory technician). More particularly, after the patient""s tooth has been shaped to receive the prosthetic crown, an impression is formed from the prepared tooth by placing impression material into the patient""s mouth (i.e., to form a negative impression of the prepared and adjacent teeth).
Once the impression has been produced by the dentist, a laboratory technician will set die pins in the impression and then form a master impression as a die (e.g., plaster models) of the patient""s teeth. The technician will set the occlusal bite registration and articulate the models of the patient""s teeth. Afterwards, the laboratory technician will saw the die to remove the tooth of interest, then trim the die of the tooth and mark the marginal finish line. The sub-structure is then waxed for preparation of the prosthetic crown.
After a wax pattern has been formed, it is converted (i.e., cast or machined) into a sub-structure (e.g., coping) of the crown. It is a challenge to produce a coping that will comply with acceptable tolerances, given the variables associated with the quality of the impression, the skill of the technician and the proper selection of die materials.
For example, U.S. Pat. No. 5,135,393, assigned to Mikrona, describes a coping mechanism for producing parts such as non-metal copings. As described therein, a three-dimensioned pattern is sensed (e.g., traced) with a feeler pin, and then sensed deflections or displacements of the feeler pin are transferred to a motor driven machining tool. As the pattern is traced, the motor driven machining tool operates upon a blank to fabricate a matching three-dimensional coping. The coping is later used by the dental laboratory to build-up a finished crown. That is, once the machined coping has been produced, it is processed with a porcelain build-up. The build-up material incorporates specific shading and color effects to simulate the enamel of the original tooth. The porcelain build-up is then vacuum fired, glazed, polished and fit.
The aforementioned ""316 application describes a technique for producing a crown without requiring the need to produce an impression in the manner described above. Rather, the ""316 application describes producing a dental prosthetic model of the crown which is to be fit to the patient""s prepared tooth. Once the model has been prepared, a dental prosthetic blank, matched in exterior dimensions to the dental prosthetic model is machined so that its interior is matched to that of the model. The entire operation can be performed in the dentist""s office, thereby eliminating any need to send an impression to a laboratory technician to produce the dental prosthetic crown.
Prosthetic dental crowns have typically been formed from a combination of metal copings having porcelain formed thereon. Other materials have also been produced and used as dental prosthetic restorations. For example, Ivoclar North America, Inc. of Amherst, N.Y. produces dental prosthetic restorations formed entirely of multi-layered resin composites. Inner layers are formed with fiber structured resin composites to give strength to the overall restoration, and then outer layers are formed of resin composites that can be more easily matched in color and shape to an actual tooth which the restoration is replacing. Due to the wear of the resin composite restorations, Ivoclar has more recently produced crowns formed entirely of ceramic. As referenced herein, xe2x80x9cceramicxe2x80x9d materials are those having increased crystalline structure, for example, crystalline structures on the order of 30% or greater. In contrast, xe2x80x9cresinxe2x80x9d or xe2x80x9cresin compositexe2x80x9d materials are those which do not possess a crystalline structure in excess of 10%. xe2x80x9cPorcelainxe2x80x9d materials are those having a crystalline structure on the order of 10% or greater. Definitions of these terms, appear, for example, in the Journal of Dental Research.
The present invention is directed to improving an apparatus for machining a dental prosthetic blank such that it is matched to a dental prosthetic model. In addition, exemplary embodiments are directed to an improved dental prosthetic blank which can be more easily, and more accurately milled to match the dimensions of the dental prosthetic model.
The present invention is directed to enhancing the accuracy with which tooth restorations are machined, using an apparatus which can accurately copy mill a dental prosthetic blank from a previously formed dental prosthetic model, and which can closely replicate the feel of a dental tool to which dentists are accustomed. In addition, exemplary embodiments are directed to the preparation of a hybrid dental prosthetic blank which can be easily machined.
Exemplary embodiments of the present invention relate to an apparatus for producing a dental prosthetic comprising: means for holding a dental prosthetic model and a dental prosthetic blank having exterior dimensions matched to those of said prosthetic model; and means for machining a three-dimensional surface of said dental prosthetic blank to match a three dimensional surface of said dental prosthetic model, said holding means being rotatable about a single axis, and said machining means having at least one arm mounted for linear rotary motion on a linear rotary axis.
Exemplary embodiments of the present invention also relate to a dental prosthetic blank which can be easily and accurately milled. In accordance with exemplary embodiments, the dental prosthetic blank comprises: a first outer material having at least a 10% crystalline structure for defining an interior cavity and having a first hardness selected as a dental prosthetic material; and a second inner resin material bonded to said interior cavity and having a second hardness, less than said first hardness, selected as a millable material. For example, the dental prosthetic blank includes an outer ceramic material (for example, Empress (trademark)I or II available from Ivoclar) or porcelain, and a second inner material formed of a resin or resin composite (such as Targis(trademark)of Ivoclar). In exemplary embodiments, the blank can be formed using an injection molding process whereby the first outer material is initially formed (for example, injection of a ceramic powder and binder into a die, followed by a sintering process). Afterwards, resin composite material can be injection molded into the outer material.